1. Technical Field
The present invention relates to a frequency synthesizer, and more particularly, to a frequency synthesizer using a wide-band voltage controlled oscillator and a fast adaptive frequency calibration method.
2. Discussion of the Related Art
An integer-N phase locked loop (PLL) has been widely used as a radio frequency (RF) synthesizer for an RF transceiver. Currently, however, a fractional-N PLL has been more widely used as an RF synthesizer for wireless communication applications.
The fractional-N PLL provides a wide bandwidth with narrow channel spacing and is free to some extent from various restrictions that are imposed on PLLs in terms of fast lock time, phase noise, and reference spurious. In addition, the fractional-N PLL provides an agile lock time and a fine frequency resolution. Therefore, the fractional-N PLL has been widely used in an RF transceiver for general packet radio services (GPRS), which generally require a fast lock time.
Many currently used wireless RF transceivers require a wide-band frequency synthesizer and a local oscillation signal generator. The local oscillation signal generator provides a variety of local oscillation signals to an RF transceiver. Currently, various methods for providing a wide frequency band to a frequency synthesizer are available. One such method is directed to increasing the gain of a voltage controlled oscillator, i.e., a ratio of the frequency to a tuning voltage. However, as the voltage of the voltage controlled oscillator increases, its phase noise performance deteriorates. In addition, in order to have a wide frequency band coverage of, for example, 500 MHz or higher with a low voltage of 3V, the gain of the voltage controlled oscillator should be increased to 200 MHz/V or higher. However, a gain of 200 MHz/V or higher is typically too much for conventional low phase noise applications to handle.